Encapsulated photonic crystal structures

ABSTRACT

Photonic crystal structures having a plurality of air columns and coating such structures to provide structures with improved performance are described herein. The coating includes a material that coats an uppermost portion of a photonic crystal structure, wherein the photonic crystal structure comprises a plurality of air columns and wherein the plurality of air columns are coated on their uppermost surface by the coating material.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/363,472, filed on Feb. 27, 2006, now U.S. Pat. No. 7,305,161 which inturn claims priority under 35 USC § 119(e)(1) to Provisional ApplicationNo. 60/656,343, filed Feb. 25, 2005.

BACKGROUND OF THE INVENTION

The present invention relates to the general field of photonic crystalsand, in particular, as photonic crystals relate to integrated circuits,optical interconnects and sensors.

Photonic crystals hold great promise for new and innovative micro- andnano-photonic and other light-emitting devices and is currently thesubject of extensive research worldwide. For example, photonic crystalshave the capability of revolutionizing the photonic industry, doing forlight what silicon did for electrons. Complete photonic integratedcircuits (PICs), including lasers, modulators, lossless bends andwaveguides, etc., may be built monolithically on the same wafer bypatterning the desired photonic crystal structure, just as integratedcircuits and lasers are now fabricated. Such mass production and highyield fabrication of PICs will have a profound impact.

Current difficulties remain in the manufacturing and processing ofphotonic crystal structures, such as how to generate cost-effectivephotonic crystal structures with submicron features. While e-beamlithography is widely used, additional patterning methods that are asfast or faster and low cost, repeatable and reliable are required forgenerating high quality submicron sized photonic crystal structures. Inaddition, there remains a need to manufacture photonic crystalstructures apart from the current follow-on microfabrication processesused to create metal contacts, etc. This is because air column latticephotonic crystals known in the art are mechanically and chemicallysusceptible to disruption, damage and/or degradation as a result of theexposed air columns on their top surface. Current practice is tocomplete all other fabrication processes before submicron featurepatterning and photonic crystal structure formation. This approach,however, limits performance because it compromises the high qualitysubmicron feature definition of the photonic crystal structure. Inaddition, the approach is not suitable for large area feature definition(e.g., PICs).

Accordingly, what is needed are improved manufacturing and processingmethods of photonic crystal structures, particularly those that enablethe creation of large area and high quality submicron features, thosethat protect the photonic crystal structure, its integrity and qualityduring processing and improve overall performance of the structure anddevice made there from.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned problems and providesphotonic crystal structures as described herein having a coating for amechanically robust structure with improved integrity and quality.

There are several benefits of the present invention. For example, thepresent invention provides high quality photonic crystal structures,such structures no longer requiring other fabrication processes, andthus no longer subject to deterioration, damage or degradation from theother fabrication processes. Without deterioration, damage ordegradation to photonic crystal structures of the present invention, theoverall performance of such structures is improved and the devices theyare useful for now exhibit significant improvements in performance. Withthe present invention, photonic integrated circuits may be manufacturedbecause of the compatibility between photonic crystal structurefabrication methods described herein and other microfabricationprocesses for such integrated circuits. Photonic crystal structures ofthe present invention are also more mechanically robust by beingprepared from a process as described herein which is simple, reliable,easy to perform, easy to replicate and is highly cost-effective.Accordingly, the present invention will lower the cost of manufacturingsuch structures and the devices they are useful for.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

FIGS. 1A-1C depict scanning electron micrographic (SEM) images of arepresentative photonic crystal structure of the present invention inwhich FIG. 1A is a top view, FIG. 1B is another top view at higherresolution; and FIG. 1C is a side view;

FIG. 2A depicts a schematic representation of a coating on the topportion of an air column of photonic crystal structure of the presentinvention;

FIG. 2B depicts another schematic representation of a coating on topportion of an air column of photonic crystal structure of the presentinvention;

FIG. 2C depicts a schematic representation of an electrical fielddistribution for a properly designed cavity in a photonic crystalstructure of the present invention;

FIG. 3A is a schematic representation of spin coating as described withthe present invention;

FIG. 3B is a schematic representation of curing as described with thepresent invention;

FIG. 3C is a schematic representation of lapping as described with thepresent invention;

FIG. 4A is a graphical representation of a photonic band gap map for a2D air column triangular lattice photonic crystal structure of thepresent invention;

FIG. 4B is a graphical representation of thickness versus photonic bandgap center frequency;

FIG. 5A is a schematic representation of a portion of a process of thepresent invention depicting a photonic crystal structure of the presentinvention with air columns in the upside down position;

FIG. 5B is a schematic representation of a portion of a process of thepresent invention depicting the application of pressure to a photoniccrystal structure of the present invention;

FIG. 5C is a schematic representation of a portion of a process of thepresent invention depicting a photonic crystal structure of the presentinvention after curing;

FIG. 6A is a schematic representation of a portion of a self assemblyprocess of the present invention depicting a particle suspensioncontacting a surface of a photonic crystal structure of the presentinvention;

FIG. 6B is a schematic representation of a portion of a self assemblyprocess of the present invention depicting one or more particles on atop surface of a photonic crystal structure of the present invention andsolution within one or more columns of the photonic crystal structure;

FIG. 6C is a schematic representation of a portion of a self assemblyprocess of the present invention depicting one or more spheres orparticles on a top surface of a photonic crystal structure of thepresent invention and the absence of solution within one or more columnsof the photonic crystal structure;

FIGS. 7A and 7B depict SEM images of a representative photonic crystalstructure of the present invention in which FIG. 7A is an image takenbefore a coating process of the present invention and FIG. 7B is animage taken after a coating process of the present invention; and

FIGS. 8A-8B depict SEM images of another representative photonic crystalstructure of the present invention in which FIG. 8A is an image takenbefore a coating process of the present invention and FIG. 8B is animage taken after a coating process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts thatcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

FIG. 1A-1C depict SEM images of an initial photonic crystal structures10 of the present invention embedded in a semiconductor substrate (whiteregion). Structures 10 comprise two dimensional (2D) air columns 12(dark regions) in a lattice arrangement. Air columns 12 exhibit a largerrefractive index contrast between semiconductor 14 and air (which istypically 3.6 to 1) and is favorable for a large transverse electric(TE) photonic band gap (PBG). High quality 2D photonic crystalstructures are typically created on as-grown semiconductor wafers(generally with a planar clean surface). No additional fabricationprocesses are required after photonic crystal structures have formed.

High quality photonic crystal structures as depicted in FIG. 1A-1Cshould remain intact when they are fabricated for a device that does notrequire any metallization (e.g., contacts) or further etching processes(e.g., for passive waveguide, coupler and splitter). Most devices (e.g.,lasers, modulators), however, require additional microfabricationprocesses, including the formation of a metal contact, mesa and/orpassivation. Unfortunately, typical photonic crystal structures, asdepicted in FIG. 1A-1C, have exposed air columns that are highlysusceptible to follow-on fabrication processes (e.g., metallization orfurther etching processes) in which the follow-on processes degradeand/or entirely damage the exposed air columns and their latticearrangement.

The present invention provides a method for protecting exposed aircolumns of photonic crystal structures. The protection is maintained inthe devices they reside in, including sensors (e.g., nano-photonicbiosensor, chemical sensor, gas sensor), ultra-compact high densitymultilayer optical interconnect systems, in which optical interconnectsprovide high speed paths between backplanes (board to board),inter-chips and intra-chip as well as high performance, low powerconsumption and ultra-small photonic devices, such as microlasers,modulators, waveguides, lossless bends, receivers, etc.

With the present invention, a coating or layer is provided that contactsa top portion of the photonic crystal structure comprising air columns.The layer provides protection to the fabricated air columns from anysubsequent fabrication processes and enables the fabrication of photoniccrystal structures as PICs, while still maintaining the integrity andhigh quality of the underlying photonic crystal structures.

Referring to FIGS. 2A and 2B, the schematics illustrate a photoniccrystal structure 200 comprising a coating 202 on its top portion 206.Photonic crystal structures of the present invention may include anyphotonic crystals structure, such as air-hole based 2D photonic crystalslab structures and planar 2D air-hole photonic crystal slab waveguidestructures. The coating may be applied in many ways, including cappingas illustrated in FIG. 2A in which the coating is in contact only witheach air column 204. In addition, the coating may be applied by alayering as illustrated in FIG. 2B in which coating 202 also appears asa thin layer on the uppermost surface of structure 200. When properlyapplied, coating 202 has minimal if any impact on any final device forwith the photonic crystal structures are used and will not affectperformance of such devices. This is because light when guided throughthe vertical cavity of air columns 204 will provide an electrical field22 distribution in the center of photonic crystal structure 200 and anyelectric field at top portion 206 (as well as bottom portion 208) willbe minimal (FIG. 2C). The coating enables an efficient electricalinjection by placing the coating contact on top of the air holes nearthe center of the light emission core. The coating further providesprotection to the fabricated air-column structure from any subsequentfabrication processes.

There are several features of a coating of the present invention. Forexample, the material useful for coating should be photopatternable andbe capable of being removed in its entirety or selectively (e.g., by wetor dry etching). In addition, the coating should exhibit a relative lowrefractive index (n˜1.5) and have a minimal impact to the photonic bandgap and defect mode. Further, the coating should transparent to thelight of interest (thereby providing minimal optical loss) and should bechemically stable so that it does not itself degrade or affect thesurface or top portion of the photonic crystal structure. Finally, thecoating should provide surface passivation. Preferred coating materialsare low cost optical, can be deposited in a highly uniform manner usingconventional techniques, are photopatternable, exhibits a goodrefractive index, do not provide surface degradation, and its thicknesscan be controlled by technique used to apply coating. Materials suitablefor coating with the present invention may be in solution. Sol-gels areparticularly suitable. As are polymers, polymer blends, silicon-dioxide,particles or spheres, gallium arsenide, indium phosphide, othersemiconductor materials, noble metals, alkali metals, earth metals,Group III metals, Group VI metals, transition metals, and combinationsthereof.

Methods of providing a coating to a photonic crystal structure of thepresent invention include growth, wafer fusion, angled sputtering, spincoating, dip-into solutions, as some examples. Those of skill in the artwill appreciate that additional alternatives may also be useful.Examples of methods for coating a photonic crystal structure of thepresent invention are further described below.

Referring now to FIG. 3A-3C, the schematics represent an example of amethod of the present invention in which the coating is applied by spincoating, similar to that used for the application of thin films as isknown to one of ordinary skill in the art. With this method, a smallamount of coating material 300 is added to the center of a photoniccrystal structure 310 (or device comprising one or more photonic crystalstructures) and structure (or device) 310 is then spun at high speed(typically at or about 3000 rpm). Centripetal acceleration will causecoating material 300 to spread and eventually reach the edge ofstructure 310 leaving a thin film of coating material 300 on the topsurface of structure 310. The thickness of the film formed by thecoating material depends on the nature of the material (e.g., viscosity,drying rate, percent solids, surface tension, etc.) and the spin process(e.g., rotational speed, acceleration, and fume exhaust). Following spincoating, the film formed by the coating may undergo curing (FIG. 3B) orlapping (FIG. 3C).

When using a spin coating method, viscous flow and evaporation of thecoating material help direct how expansive a film is formed by thecoating material. Viscosity affects radial flow of the coating material.As the coating material expands and thins, the rate of evaporation ofthe material may become more important. Additional features such ashydrolysis and condensation reactions may also be important.Importantly, with the present invention, the intrusion depth of thecoating material may be controlled based on the method used to apply thecoating.

FIG. 4A depicts a 2D air column triangular lattice photonic crystal PBGmap showing triangular lattice air-hole structures with different airfill factors. FIG. 4B depicts the impact of finite thickness in a thirddimension on PBG position, taking into account finite height on thirddimensions.

Referring now to FIG. 5A-5C, the schematics represent another example ofa method of the present invention in which the coating is applied to aphotonic crystal structure of the present invention bydip-into-solution. In FIG. 5A, a coating 502 resides on the uppermostsurface of substrate (e.g., silicon or silicon oxide substrate). Thecoating may be applied to the substrate using methods known to one ofordinary skill in the art, including a spin coating method. In FIGS. 5Aand 5B, a photonic crystal structure 506 having air columns 508 isflip-chipped and pressure pressed into coating 502. As such, structure506 is positioned such that the uppermost surface 510 is on the bottomand the bottom surface 512 is upwardly facing. Upon removal of excesscoating on the edges, the coating is cured and released from substrate504 by applying light pressure (FIG. 5B) and the structure is thenreturned to its upright position (FIG. 5C) in which structure 506contains coating 502 only in the uppermost portion of columns 508.Pressure may be adjusted in order to obtain more or less coating in theuppermost portion of columns 508.

Referring now to FIG. 6A-6C, the schematics represent yet anotherexample of a method of the present invention in which the coating isapplied to a photonic crystal structure of the present invention usingsmall spheres or particles 605 provided in a suspension. FIG. 6Aillustrates a method by which particles 605 provided in a suspension areapplied to a photonic crystal structure 610 using a spin coating methodsimilar to that described with FIG. 3A-3C. Particle size is chosen suchthat the average particle diameter is larger than the average diameterof the air columns. The solution used with the particles is typically asolvent that can evaporate with heat. Upon placing a particle suspension615 in the center of structure 610, the structure is spun allowingparticles 605 to rest on the top of air columns 620 as depicted in FIG.6B. To remove the solvent, photonic crystal structure 610 may be placedin an oven or subject to the heat (e.g., heat lamp). Structure may alsoundergo a slow spinning while the solvent is evaporated. In the end,particles 605 remain and seal air columns 620 as depicted in FIG. 6C. Byfurther providing a vacuum, particles may be partially forced in aircolumns 620 to prevent movement or later release.

The examples described above have been tested by simulation (Zhou, Appl.Phys. Lett 2006; 88:051106-1-051106-3, see pg. 051106-2 second column toand of 051106-3 first column; incorporated herein by reference) in whichwere found a large process tolerance on the coating thickness andrefractive index. In addition, FIGS. 7 and 8 illustrate additionalexamples of coating a photonic crystal structure of the presentinvention. For these examples, a 2D photonic crystal slab (PCS)structure on a silicon substrate was fabricated with e-beam lithographyand reactive ion etching. Air columns had a typical radius of about 0.13μm. The lattice constants were varied in order to optimize the coatingconditions. SEM images of 2D PCS structures having a lattice constant ofabout 0.4 μm before and after encapsulation are shown in FIGS. 7A and7B, respectively. Polystyrene spheres with a nominal (typical diameter)size of 300 nm were spin coated on top of the PCS structure, using aspin speed of 3.5 krpm and a duration of 40 seconds. After spin coating,the PCS structures were air dried. As depicted in FIG. 7B, there is anon-uniformity of polystyrene spheres 710 on the surface of the PCSstructure 720, which is due to a very high PCS r/a ratio (e.g., ratiobetween air hole/column radius r and lattice constant a). A highuniformity was achieved with a relatively low r/a ratio by applyingsmaller diameter nanoparticles or spheres to the PCS structures. Asshown in FIGS. 8A and 8B, by using smaller silica nanoparticles 810 withtypical size (diameter) of 70 nm, nanoparticles were successfullyback-filled into air columns with air hole radius of r=0.18 μm andlattice constant of a=1 μm. While a low r/a ratio PCS structure favorsmore uniform assembly of nanoparticles and, thereby coating of thematerial, a higher r/a ratio PCS structure is desirable due to itslarger gap size. Weight-volume control of particle suspension andselective surface property modification of such particles will providefor a more uniform coating of such photonic crystal structures. Both themore uniform (as depicted in FIG. 8B) and the lesser uniformityparticles or spheres (as depicted in FIG. 7B) are acceptable with thepresent invention because the primary purpose of the coating process asdescribed herein is to facilitate the metallization on top of the aircolumn (air hole) region, both of which accomplish this task.

As provided herein, the present invention includes coating methodsbelieved essential for large-scale multilevel vertical integration andhigh performance functioning of photonic crystal structures and thedevices they reside in. As described herein, such photonic crystalstructures may be useful for application to efficient electricalinjection of photonic crystal surface emitting lasers or other suchdevices, such as photonic integrated circuits. Accordingly, the presentinvention promotes the creation of robust, low-power consuming,ultra-compact high density multilayered optical interconnect systems,for use in, for example, stacked air column photonic crystalapplications, high speed application, inter-chips, intra-chips,microlasers, modulators, waveguides, lossless bends and receivers.

Although preferred embodiments have been described in detail herein, itwill be appreciated that the present invention is discussed in detailherein. It will further be appreciated that the present inventionprovides may applicable inventive concepts that can be embodied in awide variety of specific contexts. For example, it is to be understoodthat additional coating materials, such as those incorporating silicon,gallium arsenide and indium phosphide may also be used with the presentinvention. The specific embodiments discussed herein are merelyillustrative of specific ways to make and use the invention, and do notdelimit the scope of the invention. Those skilled in the art willrecognize that various substitutions and modifications may be made tothe invention without departing from the scope and spirit of theappended claims.

1. A coating for encapsulated semiconductor photonic crystal structurescomprising: a coating material that coats the uppermost portion of atleast one semiconductor photonic crystal structure, wherein the photoniccrystal structure comprises a plurality of air columns and wherein theplurality of air columns are coated on their uppermost surface by thecoating material, and wherein the coating back-fills the air columns,and wherein a metallization layer is formed above the air columns. 2.The coating of claim 1, wherein the coating is a material selected fromthe group consisting of polymer, polymer blend, silicon-dioxide,nanoparticle, gallium arsenide, indium phosphide, semiconductormaterial, noble metal, alkali metal, earth metal, Group III metal, GroupVI metal, transition metal, and combinations thereof.
 3. The coating ofclaim 1, wherein the coating is provided as a gel, sol-gel, solution,thin film, polymerizable material, curable material, photolabilematerial, photopatternable material and combinations thereof.
 4. Thecoating of claim 1, wherein the coating is nanoparticles in suspension.5. The coating of claim 1, wherein the coating material includesnanoparticles having a typical diameter that is larger than a typicaldiameter of the air column.
 6. The coating of claim 1, wherein thecoating material includes nanoparticles have a typical diameter that issmaller than a typical diameter of the air column for back-filling ofthe air columns.
 7. The coating of claim 1, wherein the coating isapplied by growth, wafer fusion, angled sputtering, spin coating, anddip-into solutions.
 8. A method of coating encapsulated semiconductorphotonic crystal structures comprising the steps of: coating anuppermost portion of at least one semiconductor photonic crystalstructure with a coating material, wherein the photonic crystalstructure comprises a plurality of air columns, wherein the plurality ofair columns are coated on their uppermost surface by the coatingmaterial, wherein the coating material includes nanoparticles insolution, and wherein the coating material back-fills the air columns,and wherein the coating facilitates metallization above the air columns.9. The method of claim 8, wherein the solution evaporates by heat. 10.The method of claim 8, wherein the nanoparticles include nanoparticleshaving a typical diameter that is larger than a typical diameter of theair column.
 11. The method of claim 8 further comprising spinning thenanoparticle suspension onto the photonic crystal structure.
 12. Themethod of claim 8 further comprising applying a vacuum to the photoniccrystal structure.
 13. The coating of claim 8, wherein the nanoparticleshave a typical diameter that is smaller than a typical diameter of theair column for back-filling of the air columns.
 14. An encapsulatedphotonic crystal structure embedded in a semiconductor substrate,comprising: a plurality of air columns; a coating material that coatsthe uppermost portion of the air columns, wherein the photonic crystalstructure is flip-chipped into the coating material, wherein ametallization layer is formed above the air columns.
 15. Theencapsulated photonic crystal structure of claim 14, wherein the coatingmaterial is a material selected from the group consisting of polymer,polymer blend, silicon-dioxide, nanoparticle, gallium arsenide, indiumphosphide, semiconductor material, noble metal, alkali metal, earthmetal, Group III metal, Group VI metal, transition metal, andcombinations thereof.
 16. The photonic crystal structure of claim 14,wherein the coating material includes nanoparticles in suspension. 17.The photonic crystal structure of claim 14, wherein the coating materialincludes nanoparticles in suspension, wherein the nanoparticles have atypical diameter that is smaller than a typical diameter of the aircolumn for back-filling of the air column.
 18. The photonic crystalstructure of claim 14, wherein the coating material includesnanoparticles in suspension, wherein the nanoparticles have a typicaldiameter that is larger than a typical diameter of the air column. 19.An encapsulated photonic crystal structure embedded in a semiconductorwafer, comprising: one or more semiconductor photonic crystal structuresembedded in the semiconductor wafer, the photonic crystal structurecomprising: a plurality of air columns; a coating material that coatsthe uppermost portion of the photonic crystal structures and theuppermost portion of the air columns, wherein the coating materialback-fills the air columns and wherein a metallization layer is formedabove the air columns.
 20. The photonic crystal structure of claim 19,wherein the coating material includes nanoparticles in suspension. 21.The photonic crystal structure of claim 19, wherein the coating materialincludes nanoparticles in suspension, wherein the nanoparticles have atypical diameter that is smaller than a typical diameter of the aircolumn for back-filling of the air column.
 22. The photonic crystalstructure of claim 19, wherein the coating material includesnanoparticles having a typical diameter that is larger than a typicaldiameter of the air column.